Studies of aerobic energy metabolism in parasites and mammals were continued and devoted primarily to elucidating the sequence of electron transfer in the respiratory chains of Entamoeba histolytica and Giardia lamblia. Efforts were focused primarily on the iron-sulfur proteins in these parasites. The subcellular distribution and properties of the Fe.S centers in E. histolytica differ from those observed in G. lamblia. Reexamination of L-serine metabolism in E. histolytica was necessary when it was found that the amebae no longer oxidize this amino acid. Detailed studies on the paradox have shown that the current strain cultivated in Kamond's new TYI-S-33 medium fail to respond to L-serine, whereas the old strain frozen since 1975 and re-cultivated in the old TP-S-1 medium, readily oxidize the amino acid as reported previously. Studies on the ATPase of Leishmania donovani kinetoplasts disclosed that these organelles readily hydrolyze ATP in the presence of Mg ion. Mammalian studies were continued on the effect of heme depletion on the respiration of cultured murine erythroleukemia cells. Walker ascites tumor cells, and isolated rat hepatocytes.